Process for impregnating fibre materials



United States Patent PROCESS FOR IMPREGNATIN G FIBRE MATERIALS BengtJohan Yngve Mossberg and Ernst Holger Berti] Nystriim, Djurshoim,Sweden, assignors to Aktiebolaget Statens Skogsindustrier, Stockholm,Sweden, a company of Sweden No Drawing. Application July 2, 1951, SerialNo. 234,918

Claims priority, application Sweden July 14, 1950 12 Claims. (Cl; 9239)The present invention concerns the impregnation of fibre materials, suchas cardboard, wallboard, chipboard and the like, in which theimpregnating agent may consist of synthetic resins both of athermoplastic character, such as polyvinylchloride, and of athermosetting character, such as aminoplast, phenoplast, alkydresin, andfurther, of raw or treated linseed oil, soya bean oil, tall oil or thelike, or compounds or combinations of the above or similar substances.

Hitherto, in the imgregnation of fibre materials the impregnating agenthas been chiefly applied either in quantities of 0.1-1.0% or inquantities of 642%, based on the dry weight of the fibre material.Quantities within the intermediate range (1-6%) have also been used,although more seldom for reasons set out hereafter. Impregnation withthe smaller quantities within the range of 0.l1.0%, such as the additionof so-called sizing medium in board manufacture, has generally beencarried out by adding the impregnating agent in the form of an aqueousemulsion while the fibre material is in aqueous suspension. An additionof the impregnating agent at this stage of the manufacturingprocess andin this manner is, however, accompanied by losses of considerableamounts of the impregnating agent which are carried away with the waterremoved in the board forming process, and this has hitherto preventedthe use of large quantities of the impregnating agent in accordance withthis method. In cases where it was desired to apply large quantities ofthe impregnating agent, therefore, it was customary to proceed in such away that the fibre material after receiving its final form was immersedin the impregnating agent, or in a solution of the same in a cheapsolvent such as kerosene.

The method of performing impregnation by a special immersion process hasmany disadvantages. It is expensive to carry out, since it requiresextensive apparatus and mechanical equipment. Furthermore, it isdifiicult to fix the content of the impregnating agent uniformly in thefibres within the desired narrow limits owing to the fact that differentparts of the same fibre material frequently absorb the impregnatingagent at different rates. When the impregnating agent is employed in theform of a solution it is easier to obtain the desired degree ofimpregnation, but in such a case the solvent must either be removed byevaporation or recovered. Evaporation of the solvent involves greatdrawbacks from many points of view, amongst others, the risk of fire,since the solvents'here in question are highly inflammable. If expensivesolvents are employed which must be recovered, this entails the use ofextensive apparatus. In many cases the desired impregnating agent mayhave such a high viscosity, however, that it cannot be used in anundiluted form. Generally, with regard to the requirement to obtain auniform impregnation and a moderate consumption of solvent, an immersionprocess is technically applicable only in cases when using the saidrelatively high contents of the impregnating agent.

2,721,504 Patented Oct. 25, 1955 The present invention provides asatisfactory solution, both from a technical and economic point of view,of the problem of impregnating fibre materials both with amounts ofimpregnating agent lying within the two above-mentioned ranges, 0.011.0%and 6-12%, respectively, and also in the intermediate range, 1.06%.

According to the invention the impregnation is carried out of a stage inthe manufacturing process at which the fibre material already forms acontinuous fibre felt but before it has attained its final form bypressing at high pressure against a polished plate or between a polishedplate and a wire net or screen, the impregnating agent being applied tothe side of the fibre felt turned towards the polished plate so that byhot pressing against the plate it is subsequently caused to penetrateinto and either become homogeneously distributed in the fibre feltthroughout its entire thickness or only to penetrate into the same to alimited depth, at the same time forcing out any water in the feltthrough the side of the latter turned away from the polished platepartly in the form of steam. After accomplished pressing the fibre boardthus treated is subjected to a heating in the presence of air at asufficiently high temperature for a sufiicient time for etfecting ahardening of the impregnating agent and the fibre material.

The process according to the invention has many advantages.

It is possible to exactly regulate and control the quantity of theimpregnating agent used for the impregnaq tion by regulating the dosagethereof. impregnating agents which are highly viscous can also beemployed with advantage, since during the following hot pressingoperation they are caused to flow to the desired extent, even withoutthe presence of a solvent. The amount of the impregnating agent appliedwhich is actually utilized for the impregnation is very high, and evenhigher under certain conditions, which appreciably reduces the costs ofthe impregnation in comparison with the processes formerly employed.

In view of the fact that the impregnating agent is caused to flowthrough the fibre material during the pressing operation, the fibrematerial will be impregnated very uniformly. Generally, the treatment iscarried out in sucha way that the impregnating agent completelypenetrates the entire mass of the fibre felt, whereby an impregnatedmaterial is obtained which has excellent properties as well regardingthe strength of the fibre material as its resistance to water. Forcertain purposes, however, it may be preferred to limit the depth of theimpregnation, and in such cases the impregnating agent is so selectedwith regard to its viscosity and added in such a quantity that under thepressure and temperature conditions adopted in pressing, it will onlypenetrate and impregnate that part of the fibre felt which is adjacentto the polished plate. In this case also an excellent result isobtained. Moreover, when working in this manner, a special advantage issecured, inasmuch as on selecting an impregnating agent which hardenswith sufficient rapidity, the impregnating agent will be concentratedmerely in the surface layer of the board which entails a reduction inthe consumption of any coating materials which may be appliedsubsequently to the board.

Especially suitable impregnating agents according to the invention areproducts of unsaturated higher fatty acidsor oils containing such fattyacids or rosin acids, such as linseed oil, soya bean oil, tall oil andthe like modified with a dienophil compound, such as maleic acid ormaleic acid anhydride or fumaric acid, and esterified with one or morepolyhydric alcohols, such as glycerol, glycol, sorbitol andpentaerythrite. A specific example of an impregnating agent of this kindis the product obv tained from tall oil by modifying it with maleicacidanhydride and then esterifying with glycerol. The composition oftall oil modified with maleic acid anhydride is not definitelyascertained but may be supposed to be according to the followingformula:

When treating this product with glycerol all acid groups therein areesterified.

As other examples of impregnating agents which may advantageously beused according to the invention may be mentioned for example linseedstand oil, phenol formaldehyde resins and urea formaldehyde resins, thetwo latter suitably in state of oil-modified products, and alkyd resins.

According to circumstances, the impregnating agent may either be appliedas such, or in the form of an emulsion, or in the form of a solution,and it may suitably be applied to the fibre felt by coating or spraying.According to a preferred embodiment of the invention the impregnatingagent is added in the form of an aqueous emulsion with water as thecontinuous phase.

If desired, two or more different impregnating agents may be used whichcan either be added in a mixture or one after the other in differentsteps.

When carrying out the invention, the treatment is adopted to theexisting conditions regarding the kind of the fibre material, thethickness and structure of the fibre felt, the kind and viscosity of theimpregnating agent, the desired content of the impregnating agent to befixed in the fibres, the temperature in the pressing operation etc.,which have a mutual relation to one another. Generally, in case of agiven fibre material, the treatment has to be carried out underconditions which regarding temperature, time and pressure lie withinvery narrow limits. The most suitable working conditions may be easilyascertained by preliminary tests.

When applying the invention in practice it has been found suitable toemploy an impregnating agent having a viscosity of from about 2000 to5000 cps. at C., in case of a complete impregnation of the fibre feltthroughout its thickness. In case of a partial impregnation of the felt,the impregnating agent may have a viscosity of from 300 to 2000 cps. at20 C., or even lower than 300 cps. The pressing of the felt afterapplication of the impregnating agent thereto is preferably carried outin one or more periods of at least to seconds at a full pressure of 15to kg./cm. at a temperature within the range of about to 220 C., thetotal period of pressing varying from about 6 to 20 minutes depending onthe temperature and pressure conditions used and the properties of thefibre material. The final hardening of the impregnated board material isthen preferably effected by heating to a temperature of about to 200 C.,for 9 to 12 hours at the lower temperatures within said range, down toabout 1 hour at the higher temperatures in the neighbourhood of 200 C.

If the impregnating medium is added in the form of an aqueous emulsionit is necessary to employ an emulsifier which gives an acid pH-reactionduring the subsequent pressing operation. Suitable examples of suchemulsifiers are ammonium compounds of fatty acids which at thetemperature prevailing during pressing are decomposed so that freeammoniac is given. off in a gaseous form.

Alkali compounds cannot be employed as emulsifiers withoutcertainmeasures, as the alkaline reactionof the water in such a case wouldcause adhesion of the fibre material to the polished plate used inpressing. However, it is also possible to use alkali compounds asemulsifiers, on the condition that in that part of the felt which duringpressing is in contact with the polished plate, the

necessary acid pH-reaction is produced in some other manner, for exampleby subsequent spraying sulphuric acid on the fibre felt before itreaches the polished plate, which however, is an inconvenient solutionfrom a technical point of view.

Substances with an additional etfectmay also be added to theimpregnating agent. Thus, in certain cases, it may be desirable toincorporate additives such as insecticides or fungicides in the boardmaterial and for this purpose such substances may be added to theimpregnating agent. As suitable examples hereof may be mentioned tetraand pentachlor phenol, chlornaphthalins etc. or their salts. Further,also colouring agents may be added.

Thus, it is highly desirable for certain purposes to give a certainshade of colour to wallboard, such as brown for example. This can becombined with an addition of substances which accelerate the hardeningfor some of the synthetic resins used as impregnating agents, such assiccatives which in the absence of other colouring pigments can impart acertain colour to the surface of the board. For example, with an ironsiccative, such as iron naphtenate, linoleate or rosinate, anapproximately grey colour is obtained, whilst the correspondingmanganese compounds give the board a colour tending towards brown. Thequantities required for producing these effects correspond to thequantities employed for obtaining the siccative efiect, that is to say,they are of the order of magnitude of 0.1-1.0% metal, based on thequantity of the impregnating medium.

The application in practice of the process according to the invention ismore particularly described below in the following non-limitativeexamples.

Example 1 In the impregnation of wallboard for the production of,so-called oil-tempered board, the oil used as impregnating agent isapplied to the upper side of the fibre felt which during subsequentpressing in a high pressure press is turned towards the polished plate,at a point immediately following the last pair of rolls of the boardmachine, or fourdrinier'forming machine, from which the fibre felt to beimpregnated is delivered. As impregnating agent, a tall oil productmodified with maleic acid anhydride and esterified with glycerine (seeabove) and having a viscosity of 2000 to 5000 cps. at 20 C. may beemployed. For entirely homogeneous impregnation of the felta quantity ofthis oil is added corresponding to 6 to 8% of the dry weight of thefibre.

The oil is added in the form of an emulsion of oil-inwater with aconcentration of 1 part oil to 7 parts water. When preparing theemulsion ammonia is added, this addition together with the free fattyacid groups of the talloil product serving as an emulsifier. Theemulsion is applied from two rows of spraying pipes provided withnozzles, whereby a perfectly uniform surface coating is produced on thefelt. When the oil emulsion encounters the upper side of the fibre feltit is broken down.

After impregnation, the fibre felt is transported into a high-pressurepress where pressing takes place at a pressure of 50 kg./cm. at atemperature of 200 C. During this pressing operation the fibre felt isfirst subjected to the full pressure of the press for a period from two,up to several tens of seconds, whereupon the pressure is reduced down tofor example 8 to 10 atmospheres and the residual water in the fibre massis allowed to boil off, whereafter the felt thus treated is againsubjected to the full pressure for a number of minutes. The totalpressing-cycle can hereby be varied within certain narrow limitsdepending upon the quality of board required. With the oil impregnationdescribed above, in order to enable the oil to penetrate the felthomogeneously, it is preferred to employ a first high-pressure period of30 to 45 seconds. Under the pressure and temperature conditions statedabove, the viscosity of the tall oil product Will decrease in such a waythat the oil isdistributed n ale homogeneously throughout the wholefibre felt whilst at the same time, owing to the temperature prevailingduring pressing, it is exposed to an initial hardening. The finalhardening is effected after accomplished pressing in hardening chambersat a temperature of, for example, 165 C., during a period of 6 to 9hours.

The oil-tempered hardboard produced in the manner described above has abending strength of 900-950 kg./ cm. and absorbs, after immersion inwater at 25 C., for 6 hours 78%, for 24 hours 14-16%, and for 72 hours21-24% of water.

Example 2 In this case the treatment is carried out as in Example 1 withthe exception that the full pressure of 50 kg./cm. is only applied forone period of 30 to 45 seconds. The pressure is then decreased forboiling off water Whereupon the board thus treated is subjected to theheattreatment for final hardening, as indicated above.

v Example 3 In order to increase the strength of a standard wallboardmaterial by 20% and improve its resistance to water an impregnation iscarried out with an impregnating agent consisting of a tall oil productmodified with maleic acid anhydride and esterified with glycerine andhaving a viscosity of 1000 to 2000 cps. at 20 C.

This oil is added in the form of an emulsion of oil in water with aconcentration of 12% oil, When preparing the emulsion ammonia is addedwhich with the free fatty acids of the tall oil product serves as theemulsifier. The emulsion is added in the same way as in Example 1,whereupon the fibre felt is subjected to pressure, likewise under thesame conditions as in Example 1. In this case, however, the oil is notdistributed homogeneously throughout the whole fibre mass but thecontent thereof decreases progressively through the fibre materialtowards the wire net side of the felt. With the form of impregnationcarried out in this way a board product is obtained which has been foundvery suitable for treatment by painting, inasmuch as the impregnatingmedium affords good adhesion for the binding medium and also gives areduced consumption of the latter.

When increasing the addition of the said impregnating agent to 2%, basedon the dry weight of the fibre material, it is possible to improvefurther the strength properties of the board. Thus, in this'case, thebending strength is increased with 50 to 60%, in comparison with theoriginal standard wallboard. 1

Example 4 In this case the treatment is carried out in the same way asin any of the previous examples, but as the impregnating agent is used aproduct obtained by modifying fatty acids from soya bean oil orcomparable unsaturated fatty acids with maleic acid anhydr'ideesterified with glycerol.

Example 5 The treatment is carried out as in any of the Examples 1 to 3but as the impregnating agent is employed blown linseed oil.

Example 6 The treatment is carried out as in any of the Examples 1 to 3but as the impregnating agent is employed, alone or in admixture with asemidrying or a drying oil, melamine formaldehyde resin etherified withbutanol, for example the product which is sold under the name Mepas.

Example 7 The treatment is carried out as in the previous example but asthe impregnating agent is employed, alone or in admixture with asemidrying or a drying oil, urea formaldehyde plast etherified withbutanol, for example the product sold under the name Karpas.

Example 8 In this case the treatment is carried out as in any of theExamples 1 to 3 but as the impregnating agent is employed, alone or inadmixture with a semidrying or a drying oil, a phenol etherified withbutanol, for example the product p-tertiary butyl phenol manufacturedand sold by the Dow Chemical Company, Midland, Michigan, U. S. A.

Example 9 The treatment is carried out as in any of the Examples 1 to 3but as the impregnating agent is used dehydrated castor oil.

Example 10 In this case the impregnation is carried out With menhadenoil. The disagreeable odour of this product disappears When the boardmaterial is subjected to the treatment for hardening.

Example 11 In this case the impregnation is carried out with a fat alkydresin dissolved in a solvent, such as white spirit. In other respectsthe treatment is carried out as in any of the Examples 1 to 3. Thesolvent is removed in the pressing operation simultaneously with thewater when boiling oif the latter.

Example 12 The treatment is carried out as in any of the foregoingexamples, but in addition a siccative is added to the impregnatingagent, such as an iron or a manganese naphtenate, in quantities of theorder of magnitude of 0.1-1.0% metal based on the quantity of theimpregnating agent.

Example 13 In this case the treatment is carried out as in any of theExamples 1 to 3 but as the impregnating agent is used blown tall oil,partly or completely esterified with a polyhydric alcohol, for examplepentaerythrite. Owing to its own colour, this tall oil product impartsto board treated therewith a grey shade of colour.

Example 14 The treatment is carried out as in anyv of the previousexamples, but in addition to the impregnating agent small amounts oftetra or pentachlor phenol, chlornaphtalins or salts of said compoundsare added.

I claim: I

1. In the manufacture of cardboard, wallboard, chipboard and similarfiberboards wherein a wet felt is pressed under conditions of highpressure and high temperature against a polished plate on one of itsfaces only, the process of impregnating the fiberboard to increase itsstrength which consists essentially in applying to one face only of thewet felt, after it has been formed into a continuous fiber felt butbefore it has received its final form by hot pressing, an aqueousemulsion having water as the continuous phase and containing from about0.1 to 12%, based on the dry weight of the felt, of an impregnatingagent selected from the class consisting of thermoplastic andthermosetting materials, said impregnating agent having a viscositywithin the range of from about 300 to 5000 centipoises at 20 C., theamount applied being suificient to cause impregnation of the felt in theamount of from about 0.1 to 12% by weight of the impregnating agentbased on the weight of the dry felt, then pressing the so-impregnatedfelt between a polished plate facing the impregnated face of the feltand a screen facing the unimpregnated face, at a temperature within therange of from about to 220 C., and at a pressure within the range offrom about 15 to SOkg-Jcm for a period of from about 6 to 20 minutes andsimultaneously driving out the'water present in the resulting boardpartly in the form of steam through the non-impregnated face, wherebythe impregnating agent is caused to penetrate to a substantial depthinto the fiberboard; the impregnated face of the felt being made acid inreaction at least prior to completion of the said pressing step, wherebysticking of the felt to the polished plate is prevented; and thenhardening the board by heating it.

2. The process of claim 1 wherein the impregnating agent consists of aproduct obtained by modifying unsaturated higher fatty acids withdienophil compound, and esterifying with a polyhydric alcohol.

3. The process of claim 1 wherein the impregnating agent is a productobtained by modifying tall oil with maleic acid anhydride andesterifying with glycerol.

4. The process of claim 1 wherein the impregnating agent is linseedstand oil.

5. The process of claim 1 wherein the impregnating agent is a ureaformaldehyde resin etherified with alcohol, in admixture with a dryingoil 6. The process of claim 1 wherein the impregnating agent is appliedin the form of an aqueous emulsion containing an ammonium compound asemulsifying agent.

7. The process of claim 1 wherein the board is hardened by heating it inair to a temperature of from about 150 to 200 C. for from about 12 hoursto 1 hour.

8. The process of claim 1 wherein the pressing operating includes atleast one high-pressure treatment followed by decreasing the pressure topermit the water contained in the felt to escape.

9. The process of claim 1 wherein the impregnating agent has a viscositywithin the range of from about 300 to 2000 centipoises at 20 C. and isapplied in quantity insufiicient for complete impregnation of the board.

10. The process of claim 1 wherein the impregnating agent has aviscosity within the range of from about 2000 to 5000 centipoises at 20C. and is applied in quantity sufficient for complete penetration of theimpregnating agent into the board.

11. In the manufacture of cardboard, wallboard, chipboard and similarfiberboards wherein a wet felt is pressed at high pressure andtemperature between a polished plate and a Wire net, the process ofimpregnating the fiberboard to increase its strength which consistsessentially in applying to one face only of the wet felt, after ithasbeen formed into a continuous fiber felt but before it has receivedits final form by hot pressing,

an aqueous emulsion of tall oil modified with maleic acid anhydride andesterified with glycerol and having a viscosity within the range of fromabout 2000 to 5000 centipoises at 20 C., said aqueous emulsion havingwater as its continuous phase, the amount applied being suffi'cient tocause impregnation of the felt in the amount of from about-6 to 8% byweight of the impregnating agent based on the dryweight of the felt,then subjecting the so-impregnated felt to a pressing operation under apressure of from about 15 to 50 l g./cm. at a temperature within therange of from about 130 to 220 C. between a polished plate facing theimpregnated face of the felt and a wire mesh facing the unimpregnatedface of the felt, thereby causing the impregnating agent to 8 flow intoand distribute itself homogeneously in the felt; the impregnated side ofthe felt being made acid in reaction at least prior to completion of thepressing step, whereby sticking of the felt to the polished'plate is"prevented; the water present in the felt partly in the form of steamthrough the said wire mesh, then hardening the board by heating at atemperature within the range of from about 150 to 200 C.

12. In the manufacture of cardboard, wallboard, chipboard and' similarfiberboards wherein a wet felt is pressed at high pressure between apolished'plate and a wire net, the process of impregnating thefiberboard to increase its strength which consists essentially inapplying to one face only of the wet felt, after it has been formed intoa continuous fiber felt but before it has received its final form by hotpressing, an aqueous emulsion of tall oil modified with maleic acidanhydride and esterified with glycerol and having a viscosity within therange of from about 300 to 2000 centipoises at 20 C., the amount appliedbeing sufficient to cause only partial impregnation of the felt with theimpregnating agent, then pressing the so-impregnated felt between apolished plate facing the impregnated face of the felt and a wire meshfacing the unimpregnated face of the felt, at a temperature within-therange of from about 130 to 220 C. and at a pressure within the range offrom about to 50 kg./cm. for a period of from about 6 to minutes whilesimultaneously removing the Water present in the felt partly in the formof steam through said-wire mesh, then hardening the board by heating itin the presence of air at a temperature withinthe range of from 7 about150 to 200 C.

References Cited in the file ofthis patent UNITED STATES PATENTS Re.19,187 Oberdorfer May 29, 1934 1,320,353 White Oct. 28, 1919 1,894,777Mason Ian. 17, 1933 1,941,536 Boehm Jan. 2, 1934 1,957,370 Thomas et a1May 1, 1934 1,988,161 Chaplin Jan. 15, 1935 2,030,625 Ellis Feb. 11,1936 2,039,243 Krzikalla et al Apr. 28, 1936 2,080,077 Howard May 11,1937 2,081,889 Borglin May 25, 1937 2,163,585 Chaplin June 27, 19392,198,269 Linzell-et-al Apr. 23, 1940 2,229,620 Bradner Jan. 21, 19412,325,302 Britt July 27, 1943 2,346,812 Bradner Apr. 18, 1944 2,398,843Muggleton et al Apr. 23, 1946 2,417,851 Young Mar. 25, 1947 2,422,345Esterberg et al June 17, 1947 2,437,931 Bergstrom et al Mar. 16, 19482,590,653. Schmutzler Mar. 25,1952

FOREIGN PATENT S 12,190 Great Britain of 1888 OTHER REFERENCES Report onThird International Conference on Timber Utilization, Paris 26-28, July1937, No. 16/ 17 Special No., p. 95.

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1. IN THE MANUFACTURE OF CARDBOARD, WALLBOARD, CHIPBOARD AND SIMILARFIBERBOARDS WHEREIN A WET FELT IS PRESSED UNDER CONDITIONS OF HIGHPRESSURE AND HIGH TEMPERATURE AGAINST A POLISHED PLATE ON ONE OF ITSFACES ONLY, THE PROCESS OF IMPREGNATING THE FIBERBOARD TO INCREASE ITSSTRENGTH WHICH CONSISTS ESSENTIALLY IN APPLYING TO ONE FACE ONLY OF THEWET FELT, AFTER IT HAS BEEN FORMED INTO A CONTINUOUS FIBER FELT BUTBEFORE IT HAS RECEIVED ITS FINAL FORM BY HOT PRESSING, AN AQUEOUSEMULSION HAVING WATER AS THE CONTINUOUS PHASE AND CONTAINING FROM ABOUT0.1 TO 12%, BASED ON THE DRY WEIGHT OF THE FELT, OF AN IMPREGNATINGAGENT SELECTED FROM THE CLASS CONSISTING OF THERMOPLASTIC ANDTHERMOSETTING MATERIALS, SAID IMPREGNATING AGENT HAVING A VISCOSITYWITHIN THE RANGE OF FROM ABOUT 300 TO 5000 CENTIPOISES AT 20* C., THEAMOUNT APPLIED BEING SUFFICIENT TO CAUSE IMPREGNATION OF THE FELT IN THEAMOUNT OF FROM ABOUT 0.1 TO 12% BY WEIGHT OF THE IMPREGNATING AGENTBASED ON THE WEIGHT OF THE DRY FELT, THEN PRESSING THE SO-IMPREGNATEDFELT BETWEEN A POLISHED PLATE FACING THE IMPREGNATED FACE OF THE FELTAND A SCREEN FACING THE UNIMPREGNATED FACE, AT A TEMPERATURE WITHIN THERANGE OF FROM ABOUT 130* TO 220* C., AND AT A PRESSURE WITHIN THE RANGEOF FROM ABOUT 15 TO 50KG./CM2 FOR A PERIOD OF FROM ABOUT 6 TO 20 MINUTESAND SIMULTANEOUSLY DRIVING OUT THE WATER PRESENT IN THE RESULTING BOARDPARTLY IN THE FORM OF STEAM THROUGH THE NON-IMPREGNATED FACE, WHEREBYTHE IMPREGNATING AGENT IS CAUSED TO PENETRATE TO A SUBSTANTIAL DEPTHINTO THE FIBERBOARD; THE IMPREGNATED FACE OF THE FELT BEING MADE ACID INREACTION AT LEAST PRIOR TO COMPLETION OF THE SAID PRESSING STEP, WHEREBYSTICKING OF THE FELT TO THE POLISHED PLATE IS PREVENTED; AND THENHARDENING THE BOARD BY HEATING IT.